Add i18n-tasks for missing/unused translations

* i18n-tasks looks for missing *or* unused translations
* Add config file to ignore certain i18n keys (eg: activerecrord) which
  are used as defaults but not explicitly in views or helpers
* Remove code to raise in test env when there are unused translations
Looking at the [docs](http://rubydoc.info/gems/i18n-tasks#RSpec_integration), it seems like they have a template we can use:

    cp $(i18n-tasks gem-path)/templates/rspec/i18n_spec.rb spec/

I still don't love the idea of copying that template into every project, but at least we wouldn't have to maintain it in Suspenders. The template is pretty similar to what you have.

My gut is that the best way to proceed would be:

* Change this pull request to copy the template from i18n-tasks instead of keeping a copy in Suspenders.
* Make a pull request on i18n-tasks which makes it easier to pull these in, either as rake tasks or shared example groups.

Thoughts?